Rapid advances in the electronic industry, triggered by the use of very large scale integration (VLSI) components, have created new problems in electromagnetic interference and radio frequency interference (EMI/RFI) containment. Computer system using VLSI chips and reduced instruction set computer (RISC) architectures may realize short instruction cycles by using high frequency clocks. Clock frequencies in the range of 100 MHz to 200 MHz are now common in high performance computers. Digital circuits operating at these frequencies may have pulse rise times of less than 2 nanoseconds causing EMI/RFI to be radiated at harmonic frequencies in excess of 2 GHz. EMI/RFI containment problems at high frequencies intensify because the relatively short wavelength radiation "squeeze out" or passes through smaller openings in the equipment enclosure (chassis) and requires more expensive and complex filters on the connectors for cables to peripheral equipment. Minimizing EMI/RFI is one of the most difficult aspects of designing high speed computers. Indeed international and FCC regulations set strict EMI/RFI standards for certain categories of electronic equipment.
The problem of EMI/RFI containment is of particular importance when computer systems incorporating high speed logic, such as desktop personal computers and workstations, are used in an office setting. In such systems, a single printed circuit board (PCB), mounted within a chassis, customarily serves as a so called motherboard. The pins of the various components, such as the power supply, the central processor unit (CPU) chip, high speed logic, memory and option card connectors, attach to power, ground and signal traces etched on the various conducting layers of the PCB. Each system may also have input/output (I/O) interfaces for communicating with peripheral equipment such as printers, and other computer systems. The peripheral equipment is connected to the I/O interface by cables that attach to an I/O connector mounted near the edge of the PCB and partially protruding through the chassis. Although, it is relatively easy to construct a shielded chassis, designing a shielded I/O connector is relatively difficult.
Each I/O connector is a potential transmitter of EMI/RFI. The EMI/RFI is propagated to the I/O connector via the circuits connecting it with the high speed logic. The EMI/RFI is also picked up directly from radio frequency signals radiated by the high speed logic within the chassis environment (near-field electromagnetic radiation). Traditional EMI/RFI containment techniques for an I/O connector have resulted in complex and expensive connector and chassis designs including embedded capacitive filters, shielded cables and pins encased in a metallic housing.
The cost of providing effective non-interfering I/O connectors for high performance systems has recently become a greater proportion of the total system cost. For example, shielded I/O connectors may cost as much as ten times more than comparable off-the-shelf, mass produced, molded plastic connectors. It is desirable that computer systems be easily and economically constructed using low cost, unshielded and unfiltered I/O connectors, without adversely affecting EMI/RFI containment.